There exist a number of methods for determining the refractive index of optical materials. If the material is in the form of a well-defined geometric shape with straight sides, such as a block or a prism, and the sides are polished, then a classic method is to measure the critical angle of a ray of light refracted into the block. The refractive index of the material is then related to the critical angle by means of the equation sin .theta..sub.c =1/n, where n is the refractive index to be measured, and .theta..sub.c is the critical angle. If the material is in the form of a block of known thickness, then a traveling microscope can be used to measure the optical thickness, which is known to be 1/n times the actual measured thickness. The refractive index n can thus be determined.
In the characterization of lenses, it is often necessary to determine the refractive index of the lens material. The above methods cannot be used for a lens, or for any irregularly shaped object, since the material to be measured does not have a well-defined straight-sided geometric shape. Early methods for lenses and other irregular-shaped objects include that of immersing the object into a known mixture of fluids, and adjusting the mixture components until the object seems to disappear, which occurs when it has the same refractive index as the fluid. A knowledge of the fluid composition enables its refractive index to be determined from known tables. Fluids suitable for this method, include organic fluids such as carbon disulfide, benzene and nitrobenzene.
For a lens, another method is to measure its focal length by one of the classical methods, and to mechanically measure the radii of curvature of its two surfaces using a spherometer. The refractive index n of the lens material can then be calculated using the well-known lens formula: EQU 1/f=(n-1) [1/r.sub.1 -1/r.sub.2 ] (1)
where f is the focal length, and r.sub.1 and r.sub.2 are the radii of curvature of the surfaces.
This method cannot be used in situations where the use of a spherometer is inconvenient, or, if mechanical contact could damage the lens surface, or, if the lens material is not rigid enough to withstand mechanically deformation during a spherometric measurement. All of these situations may be likely in the case of very small lenses such as ophthalmic contact lenses.
More modem methods have been proposed using high speed electronic circuits and optoelectronic generators, wherein the time of flight of a beam of light over a particular path is compared, with and without a lens in the beam path. The slight delay caused by the increased optical path length through the lens, enables the refractive index of the lens material to be obtained. Such apparatus is very expensive, and thus unsuitable for widespread use.
An alternative and cheaper non-contact method is that of immersing the lens in a liquid bath, contained in an immersion cell, and measuring its focal length with and without the liquid, using a focimeter or lens power meter. Applying the lens equation (1) for the two cases, once with the multiplicand (n-1) when the lens is unimmersed, where the "1" approximates the refractive index of the air surrounding the lens, and once with the multiplicand (n-n.sub.1), where n.sub.1 is the refractive index of the liquid in which the lens is immersed, it is possible to calculate the unknown refractive index. The result of this calculation is that EQU f.sub.2 /f.sub.1 =(n-1)/(n-n.sub.1) (2)
where f.sub.2 and f.sub.1 are the measured focal lengths with and without the immersion liquid. Using either a manual focimeter, or an automatic power meter, the unknown refractive index n of the lens can be easily obtained. This method has been widely used in the ophthalmic industry for many years, despite the inconvenience of handling and drying the lens after fluid immersion. In some cases, immersion of the lens may be forbidden, and this method cannot then be used.
In German Patent No. 3724001 to W. Vieweg, hereby incorporated by reference, there is described a variation of the fluid immersion method, wherein the two lens surfaces are sandwiched between two layers of a soft flexible transparent plastic material, such as RTV, which effectively "immerse" the lens in the plastic medium during measurement. This method uses exactly the same calculations as the classical fluid immersion method, the end result of which is shown in equation (2) above, but has an advantage of greater convenience since no fluids are involved. The layers of the soft flexible transparent plastic material are in the form of pads, with the side remote from that which contacts the lens surface being attached to a flat rigid transparent sheet of glass or plastic material, through which the light enters and leaves the "immersion cell".
Though this method would appear to be an exact "dry" equivalent of the fluid immersion method, it does have one serious disadvantage which makes it very difficult to apply in practice. In the fluid immersion method, the effect of gravity ensures that the light enters and leaves the immersion cell at the same angle, ideally at normal incidence, and thus symmetrically centered with respect to the lens under test. With the above prior art method of using two soft plastic pads, there exists a serious problem to ensure centralization and symmetry of the optical path through the "immersion cell" since the outer rigid surfaces of the pads cannot be kept accurately parallel. When the pads are forced into contact with the lens surface by means of mechanical pressure, there is a tendency for them to skew or shear sideways. In the above mentioned patent, jigs are used to try to ensure maintenance of centralization, but even this is not completely effective since even if the top and bottom rigid surfaces are constrained to be normal to the beam direction, the lens itself is supported only by the soft pliable plastic pads, and is thus free to move both axially and angularly. Once axial symmetry is lost, the measurement rays passing through the lens-pad assembly are no longer paraxial, therefore introducing aberrations and measurement inaccuracies.
There is another serious disadvantage of the double-sided method using pads with flat outer surfaces, as described in the above cited prior art. The described method and apparatus cannot be used in such cases where the combination of the flexible pad and the lens have zero power, since the sensitivity of the measurement is then zero.
Likewise, with lenses of very low power, the sensitivity is very low, and the resulting measurement inaccurate. Thus, despite its apparent simplicity, this method has not found acceptance in the ophthalmic industry, possibly because of the above mentioned problems in use.
Most of the above mentioned methods are time consuming, or require the use of expensive instrumentation, or are inconvenient to apply, especially those involving immersion of the lens in a fluid bath. The last described method, using two soft flexible plastic pads, though convenient and simple, suffers from an accuracy problem, which has not been solved in practice. There therefore exists a serious need for an instrument for measuring the refractive index of a lens, which is of low cost, enabling it to be widely used in ophthalmic practices, and which is convenient and speedy to use.